The present invention relates to an apparatus for dividing pulp flows in a pulp or paper production process.
The flow of medium-consistency pulp is regulated mainly by means of a flow meter and a pulp flow valve. The flow meter is located in a pipeline prior to or after the pulp flow valve and the flow is regulated by changing the opening angle of the valve. This kind of pulp flow regulation is usually reliable and ensures the exact desired flow rate in all situations.
If pulp is to be splitted into two separate flow channels, it is typical that the splitting is effected by arranging two flow measurements and two regulation valves in parallel so that the desired flow may be set in both pipe lines, either independently of each other or by connecting the regulators to each other so that the regulators distribute the pulp into the desired pipe lines. With this arrangement, all process solutions relating to pulp distribution may be realized reliably and the regulators may be separated from each other and controlled independently.
However, there are several mill applications where absolutely exact distribution between the pipelines is not needed. The distribution may be considered successful, if the pulp is divided with an accuracy of at least 40/60% between the pipelines and there is no need to change the distribution ratio of the pulp in the process, but it is typical for the process in its continuous operation. This kind of situation prevails in several pulp washing apparatuses, e.g. in the DrumDis-placer®-drum washer, wherein the pulp is fed onto the washer drum through a feed box extending to the whole length of the drum. The pulp entering the washer is distributed into two or more pipelines, via which the pulp is led to various locations in the pulp feed box, but finally the pulp enters one and the same hydraulic space. Thus, the final equalizing of the pulp takes place only after the distribution and in view of the process this equalizing of the distribution is adequate.
At low consistency, less than 6%, the properties of pulp are similar to those of liquid, whereby regulation for distributing the pulp into different pipelines may be carried out by means of suitable pipeline design using e.g. hydraulic elbows, T-branches, reductions and other suitable pipe fittings. In these, the flow resistances are designed so that after the distribution the pulp encounters an equal flow resistance in both branches, whereby the flow resistances do not guide the distribution of the pulp. Pipe fittings, in which mixing is effected by means of reduction/enlargement of the pipe can be used only, if the amount of material flowing into each pulp line is the same, but absolutely exact distribution between the lines is not required. This kind of distribution systems have been used e.g. in feed pipings for vacuum drum filters and feed pipings for low-consistency-fed drum washers, e.g. DrumDisplacer®-washers. Additionally, the same system has been applied in feed pipings for pulp screening plants, where the pulp to be distributed is to be made to mix in the distribution point and thus fed to two different devices in as similar form as possible.
Distribution of medium-consistency (6-16%, especially 8-14%) pulp has mainly been carried out using piping provided with flow measuring and a valve, and automation. In some systems, the distribution of the pulp has been boosted e.g. by means of turbulence-generating members or piping design, but the main regulation has been effected by means of a valve and flow control. This has been a typical constructional solution, because medium-consistency pulp flow causes such a flow resistance that turbulence generated by the distribution of the pulp by means of piping is as such not adequate for maintaining the mixing required for the distribution, but as a rule, flow resistances after the distribution point effect the distribution. If the pulp flow stops in a flow channel or pipe after the distribution point, the force required for compensating for the static friction is even in a short piping so great that it is not possible to enforce the flow, but the pulp keeps flowing via the other channel only, despite the greater flow resistances. Flow properties of medium-consistency pulp have been described by e.g. Gullichsen et al., Tappi, 64 (1981) No. 6, p. 69-72.
In order to enable uniform feeding of medium-consistency pulp into an apparatus, e.g. into said washer feed box, the feeding is to be effected via more than one feed pipe, into which pipes the pulp is distributed from the main line. In such a case, each feed pipe typically has to be provided with a regulation valve and a flow meter, as described in the above. The flow meters dose the same amount of pulp into each feed pipe, and the desired flow volume is ensured by a proper position of the valves. This kind of arrangement is needed, because none of the feed pipes may get clogged, but continuous operation of all the pipes is inevitable. As the apparatus size increases, this results in a large number of regulation valves and flow meters, which thus involves an increase in costs.
Our new studies have revealed that an exactly equal flow volume in every feed pipe is not absolutely inevitable. This is true especially when the pulp is ultimately led from the feed pipes into one and the same undivided space, wherein the pulp flow will equalize anyway. Thus, the distribution of the pulp into the feed pipes does not have to be so uniform that it would require regulation by flow meters, but a resulting problem is the clogging risk of the feed pipes without flow control.